The invention relates primarily to microwave communication systems wherein data is communicated from point to point over a plurality of channels at microwave frequencies. Such communications systems are frequently used to send computer data from one computer terminal or center to another as well as for other digitized data and video and audio signals. Many channels are customarily provided within the same microwave link. Both digital and analog modulation techniques have been used in prior art systems, digital being the more reliable of the two and presently being almost exclusively used for communications between computer terminals.
QPSK (quaternary phase shift keying) has been used in some installations to both increase the number of bits of data per second which may be transmitted over a fixed bandwidth as well as for improving the maximum signal-to-noise ratio for a given channel and bandwidth and spacing between microwave transmitter and receiver positions.
Diversity transmission and reception is commonly employed where signals are subject to multipath distortion. Such distortion occurs when the transmitted signal arrives at the receiving antenna by a plurality of paths and instead of directly from transmitting antenna to receiving antenna. Fading occurs when the signals arriving at the receiving antenna add out of phase with one another effectively cancelling one another. The amount of multipath distortion varies from time to time as atmospheric conditions change.
Diversity transmission and reception alleviate these problems to a large degree as signals of horizontal and vertical polarization are affected differently by different atmospheric conditions as well as by the different features on the terrain between two stations.
In such diversity systems, at the receiving station two antennas apart from one another are used although both may be mounted upon the same tower. Signals with horizontal polarization are received with one of the antennas while signals of vertical polarization are received with the other one of the antennas. At the transmitting station, the signals are transmitted with both polarizations. A clock signal may be transmitted along with the data in each channel. At the receiving end, a decision is made as to which of the horizontal or vertical received signals is the better and that signal selected as the output of the system.
In prior art systems, the switch from one data stream to the other was made without regard to the relative phase between data in the two data streams. The result was that frequently data was lost when switching from one data stream to the other. This made the overall system reliability much lower than it would be if switching could have been accomplished without the loss of the data. To compensate for the reduced reliability of the system, it was frequently the practice to either increase the transmitter power or decrease spacing between stations.